The Institute of Electrical and Electronics Engineers (IEEE) inter alia sets the standards for communication devices interchanging information using the Ethernet protocol in order to enable for example different manufacturers to produce devices complying with the same specifications and thus being compatible to each other. For example 10BT is a well known Ethernet standard protocols for transmitting digital information at a transmission speed of 10 Mbit/s, 100BT defines digital data transmission at 100 Mbit/s and 1000BT defines the transmission at 1000 Mbit/s=1 Gbit/s.
The IEEE 802.3 standard defines the requirements for the combined 10BT/100BT/100BT transmitters using unshielded twisted pair (UTP) lines. That is IEEE 802.3 for example defines what voltage levels should be output on the lines, how the switching between the different voltage levels defined for the protocols should be handled and what termination impedance should be guaranteed on the line.
For the transmission protocols different requirements are specified in the standard. For example the highest transmitter linearity is required in 1000BT in the presence of an interferer put on the line in full duplex. The highest accuracy of signals is required in 100BT mode when fast settling with accurate rise time and accurate output voltage are specified. The 10BT protocol requires the highest voltage swing.
Conventional solutions for a combined transmitter being able to output signals according to all three protocols use a single complex current-mode digital-to-analog converter (DAC) to shape the edges of the 100BT and 1000BT waveforms and provide a very accurate equalization in 10BT mode. It is known that meeting the defined requirements of transmission speed, signal accuracy, linearity and output voltage swing with the same transmitter proves costly in terms of power consumption. To meet the 100BT/1000BT amplitude requirements and to keep the proper line termination, for example by means of a resistor parallel to the driver the current-mode DAC need a full-scale current of 40 mA. If the driver is operated in 10BT mode the same current-mode DAC needs a full-scale current of 100 mA to meet the specifications of this protocol, which specifies larger signal amplitude, namely 5V peak-to-peak, for 10BT. The specification of the large amplitude in 10BT furthermore requires a common voltage at the DAC output of at least 1.8 Volts. The power consumption of the transmitter thus results in 72 mW when operating in 100BT or 1000BT mode and in 180 mW in 10BT mode. Furthermore the list of materials needed for the final assembled product is increased by a low dropout regulator (LDO) located off the chip, which supplies a center-tap current for the current-steering DAC.
Following from the afore mentioned specifications there is a need for a transmitter capable of producing output signals according to the 10BT and the 100BT and the 1000BT specifications with low power consumption on a small silicon area.